This invention relates to an FM receiver and, in particular, to an FM receiver that has a signal transmission capability.
In an FM receiver, a leakage portion of the local oscillating signal generated by the receiver local oscillator may be radiated from the receiver antenna as a spurious wave. Generally, this spurious wave radiation can be exploited for signal transmission by modulating the frequency of the local oscillator with an external signal. For example, the FM receiver can be used as an FM transmitter by modulating the local oscillator signal with an audio information signal.
FM receivers having automatic frequency control circuits (AFC) are known. Such AFC circuits control the frequency of the local oscillator and the FM receiver incorporating such circuits usually includes an AFC switch for the selective utilization of this feature. If it is desired to provide such an FM receiver with a signal transmission capability, it is necessary generally to include two more manually operable switches. One of these additional switches is used to change the FM receiver from a signal receiving mode to a signal transmitting mode and, of course, to the signal receiving mode. The other additional switch is used to turn off the transmitted signal, as by preventing the local oscillator from being modulated by the audio signal to be transmitted. However, where the limitation of space is a primary factor in the construction of such FM receivers, and in particular where the FM receiver is combined with a magnetic tape cassette recording/playback device, the use of three manually operable switches requires a significant portion of available space. In addition, the use of such three switches tends to increase the cost of production of the FM receiver.